Purpose: To evaluate the clinical potential of diffusion-weighted MR imaging with apparent
diffusion coefficient (ADC) mapping for the assessment of gastrointestinal stromal
tumour (GIST) response to targeted therapy in comparison with 18F-FDG PET/CT
Methods and Materials: Five patients (3 W/2M, aged 56±13 y) with metastatic
GIST underwent both a 18F-FDG PET/CT (Discovery LS, GE Healthcare) and a
MRI (VIBE T1 Gd, DWI [b = 50,300,600] and ADC mapping) before and after
change in therapy. Exams were first analysed blindly and then PET/CT images
were coregistered to T1 Gd MR images for lesion detection. SUVmax and ADC were
measured for the six largest lesions on MRI. The relationship between SUVmax and
ADC was analysed using Spearman's correlation.
Results: Altogether, 24 lesions (15 hepatic and 9 non-hepatic) were analysed on
both modalities. Three PET/CT lesions (12.5%) were initially not considered on ADC
and 4 lesions on the second PET/CT were excluded because of hepatic vascular
activity spillover. SUVmax decreased from 7.2±7.7 g/mL to 5.9±5.9 g/mL (P = 0.53)
and ADC increased from 1.2x10-3 mm2/s ± 0.4 to 1.4x10-3 mm2/s ± 0.4 (P = 0.07).
There was a significant association between SUVmax decrease and ADC increase
(rho= -0.64, P = 0.004).
Conclusion: Changes in ADC from diffusion-weighted MRI reflect response of
18F-FDG-avid GIST to therapy. The exact diagnostic value of DWI needs to be
investigated further, as well as the effect of lesion size and time under therapy
before imaging. Furthermore, the proven association between SUVmax and ADC
may be useful for the assessment of treatment response in 18F-FDG non-avid GIST.